1. Field of the Invention
This invention relates generally to the field of displays, and more particularly to enhancing the visual performance of transmissive displays that utilize a transparent slab waveguide to provide light to the pixel shuttering mechanisms that perform image modulation on the display surface.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art that may be related to aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Most commonly used displays include an array of electro-optic elements, also referred to as an array of pixels, that generate light in response to an applied voltage or current. For example, liquid crystal displays, or “LCD displays”, include an array of pixels that are formed of a molecular layer deployed between two transparent electrodes. The molecular layer lights up when a voltage is applied across the two transparent electrodes. For another example, plasma displays include an array of pixels that include an inert gas contained between glass panels. Applying electric field to the inert gas turns it into plasma that emits light. The pixel array in these displays includes equal numbers of red, green, and blue pixels that produce color images when the appropriate control signals are applied to the pixels.
An alternative to the conventional displays described above is a flat panel display that uses Frustration of Total Internal Reflection (“FTIR”) to extract light traveling inside a waveguide. Typical FTIR display systems include a transparent rectangular slab waveguide that acts as a light source for an overlaying array of pixels. In operation, light (usually monochromatic light) is edge-injected into the waveguide at a selected angle such that the injected light is totally internally reflected by the surfaces of the waveguide. In some cases, the waveguide is mirrored on one or more of the side edge surfaces of the slab waveguide to increase the number of transits of the total internal reflectance (“TIR”) light rays traveling within the waveguide. Light traveling within the waveguide can then be locally, selectively, and controllably extracted at each pixel location by frustrating the total internal reflection of light bound within the waveguide.
Various flat panel FTIR-based display systems have been developed over the last several decades. Among them is the Time Multiplexed Optical Shutter (TMOS) disclosed in U.S. Pat. No. 5,319,491 by Martin G. Selbrede (hereinafter referred to as “Selbrede '491”), which is incorporated by reference in its entirety herein, as well as variations disclosed in, for example, U.S. Pat. Nos. 7,092,142, 7,042,618, 7,057,790, 7,218,437, 7,486,854, and 7,450,799, which are also incorporated by reference in their entirety herein.
An example of an FTIR-based optical display, described in Selbrede '491 and other patents mentioned above, is a flat panel display that includes a plurality of pixels distributed across a planar slab waveguide. One example of such a display is depicted in FIGS. 1A and 1B. FIG. 1A is a basic cross-sectional schematic of two adjacent pixels 100, 105 of a display. A slab waveguide 130 is formed of a material having a refractive index higher than the square root of two, namely, 1.4146, and has a reflective coating (not shown) on the slab edge farthest from a light source injection edge 132. An electronics layer 112 may be disposed on a top surface 133 of the waveguide 130 to provide electronics that selectively control and actuate each of the individual pixels. Electronics layer 112 includes driver electronics, such as TFTs 114 and conductive interconnects 116 (described in more detail below), and an electronics layer pixel conductor 115, 150 at each pixel location in order to selectively actuate each pixel. The electronics layer pixel conductor 115, 150, hereinafter referred to simply as a “pixel conductor”, is preferably formed from a transparent conductor material, such as indium tin oxide. An active layer 118 is supported parallel to and spaced-apart from the waveguide 130 by a plurality of spacers 110 that are positioned on a top surface 125 of the electronics layer 112. The active layer 118 may comprise a thin sheet of polymeric material 120 (e.g., an elastomer), a common conductor 122 that spans across any number of pixels disposed on a display, and a nonconductive light coupling layer 123. The sheet of polymeric material 120 is elastically deformable and preferably comprises a sheet (or film) of transparent elastomeric material that has a high refractive index selected to optimize the coupling of light when extracting light via FTIR. The common conductor 122 is preferably formed from a transparent conductor material, such as indium tin oxide. The nonconductive light coupling layer 123 may comprise, for example, a plurality of optical microstructures (e.g., a microlens array) to facilitate light extraction from the waveguide 130 and direct the extracted light towards a viewer. The plurality of spacers 110 position the active layer 118 in a spaced-apart relationship to the top surface 125 of the electronics layer 112, thereby forming a microscopic gap 135 (e.g., vacuum or gas-filled gap) therebetween. Spacers 110 are distributed across the top surface 125 of the electronics layer 112 at the periphery of the individual pixel geometries such that the active layer 118 is tethered by the spacers 110 surrounding each pixel location. In one example, the spacers 110 are formed of a square grid of aluminum disposed onto the top surface 125 of the electronics layer 112 such that each square of the grid surrounds a single pixel 100, 105 location. Thus, each pixel 100, 105 represents a discrete subsection of the display, wherein each pixel 100, 105 comprises a portion of the active layer 118 and a portion of the electronics layer 112 delineated by the spacers 110 surrounding each pixel location.
In operation, field sequential color light (e.g., sequentially illuminating primary color lights such as red, green, and blue) is edge-injected into the slab waveguide 130 and undergoes total internal reflection (“TIR”) within the waveguide 130 and layer(s) (e.g., electronics layer 112) disposed thereon having a similar refractive index. The edge-injected light undergoes TIR at the interfaces of a low refractive index cladding layer formed by the air gap 135 and air surrounding a bottom surface 134 of the waveguide, thereby trapping TIR light waves 145 inside the waveguide 130 and layer(s) disposed thereon. For example, as depicted in FIG. 1A, TIR light waves 145 are constrained by the air interfaces at the top surface 125 of the electronics layer 112 and at the bottom surface 134 of the waveguide 130 due to the presence of the low refractive index air-filled gap 135 (i.e., a cladding layer) adjacent the top surface 125 and air surrounding the bottom surface 134.
FIG. 1B conceptually illustrates an OFF state and an ON state of the two pixels 100, 105, respectively, that are adjacently situated and separated by one or more spacers 110. Pixel 100 illustrates a single pixel in a non-actuated position, also referred to as an inactive state or a quiescent state. In the quiescent state, a portion of the active layer 118 between the spacers 110 associated with the pixel 100 is situated parallel to the top surface 125 of the electronics layer 112 and separated from the top surface 125 by the microscopic gap 135 such that no light is emitted at that pixel location (i.e., the pixel 100 is in an OFF state). For example, a bottom surface 140 of the active layer 118 is preferably separated from the top surface 125 by a gap height (h) (i.e., distance) greater than about 200 nanometers (e.g., a height in a range from about 200 nm to 6000 nm or greater) to insure that essentially no coupling of TIR light waves 145 from the waveguide 130 to the active layer 118 occurs across the gap 135.
Pixel 105 illustrates a single pixel in an actuated state, also referred to as an active state or an ON state position. The pixel 105 is switched to an ON state by electrically charging the pixel conductor 150, as indicated by the crosshatching of conductor 150 in FIG. 1, so as to create a sufficient potential difference and concomitant electric field between the common conductor 122 and the electronics layer pixel conductor 150 that causes the common conductor 122 and elastomer layer 120 attached thereto (i.e., the deformable active layer) to deform and move towards the electronics layer conductor 150 such that the active layer 118 moves into (or nearly into) contact with the top surface 125 of the electronics layer, thereby frustrating the TIR light within the waveguide. Thus, activation of the electronics layer conductor 150 selectively controls and actuates a portion of the active layer 118 in the area of the pixel 105 delineated by the surrounding spacers 110 by locally propelling the deformable active layer 118 across the microscopic gap 135 and into contact (or near contact) with the top surface 125 of the electronics layer 112 such that TIR light waves 145 are frustrated and emitted light waves 155 are directed out of the waveguide 130 and released from the active layer 118 at the actuated pixel location.
The electronics layer 112 typically includes driver electronics comprising thin-film transistors (“TFTs”) 114 such as active matrix thin-film transistor (“AM-TFT”) structures and conductive interconnects 116 such as metallic electrical traces. The driver electronics are utilized to electronically switch the individual pixels 100, 105 between ON an OFF states. For example, each pixel 100, 105 may include an AM-TFT device 114 that applies an appropriate voltage to pixel conductor 115, 150 that creates a sufficient electrical potential difference (ΔV) across the gap 135 (i.e., between the pixel conductor 115, 150 and the common conductor 122) so as to generate an electric field that causes deformation and high-speed motion of the active layer 118 towards the waveguide 130, as previously described with respect to pixel 105. To switch the pixel's state to OFF, the AM-TFT device 114 switches the voltage applied to the pixel conductor 115, 150 to a suitable voltage that sufficiently reduces the electrical potential difference (e.g., ΔV=0) between the pixel conductor 115, 150 and the common conductor 122 such that the deformed active layer 118 can elastically return to its non-deformed parallel orientation in the pixel's quiescent OFF state (previously described with respect to pixel 100).
The plurality of pixels 100, 105 in an FTIR display are electrostatically controlled micro-electro-mechanical systems (“MEMS”) structures that controllably deform and propel the active layer 118 across the microscopic gap 135 into contact or near-contact with the top surface 125 of the electronics layer 112 such that light transits from the waveguide 130 to the active layer 118 either by direct contact propagation and/or by way of evanescent coupling. Each pixel 100, 105 can therefore be actuated to the ON position by propelling the active layer 118 across the microscopic gap 135 by electromechanical and/or ponderomotive deformation of the active layer 118. Application of an appropriate electrical potential across the gap 135 between the pixel conductor 115, 150 associated with the slab waveguide 135 and the common conductor 122 associated with the active layer 118 causes the deformation and high-speed motion of the active layer 118 toward the waveguide 130. Actuation is deemed completed when the active layer 118 is in physical contact (or in near contact) with the top surface 125 such that TIR light 145 can be coupled out of the waveguide 130 via FTIR. The intensity of the extracted and emitted light 155 can be controlled by either pulse width modulation (PWM) (i.e., digital grayscale) of the applied voltage or by varying the magnitude of the applied voltage (i.e., analog grayscale). Furthermore, the contact/near-contact events can occur at very high speeds in order to permit the generation of adequate gray scale levels for multiple primary colors (e.g., field sequential color light) at video frame rates and in order to avoid excessive motional and color breakup artifacts while preserving smooth video generation.
Thus, as described above, the pixel operating mode described in Selbrede '491 and the other patents mentioned above involve a pixel architecture in which the mechanically quiescent (i.e., non-actuated) position of the pixel is the pixel's OFF-state position (e.g., the quiescent pixel 100 in FIG. 1B, and both pixels as depicted in FIG. 1A), and the actuated position of the pixel is the pixel's ON-state position (e.g., the actuated pixel 105 in FIG. 1B). In the quiescent position, the pixel 100 is optically inactive because the active layer 118 is in a spaced-apart relation to the slab waveguide 130 such that light coupling (e.g., evanescent coupling) across the gap 135 is negligible. In the actuated position, the pixel 105 is optically active because electromechanical or ponderomotive deformation of the active layer 120 brings the active layer 118 physically across the gap 135 and into contact or near-contact with the slab waveguide 130 or electronics layer 112, thereby causing FTIR which allows light to be emitted from the actuated pixel region 105.
However, the imposition of the driver electronics 114, 116 and pixel conductor 115, 150 within the electronics layer 112 as disposed over the waveguide 130 can have deleterious optical effects. In particular, deploying driver electronics based on TFTs in an active matrix context (AM-TFT drive mechanisms) within the electronics layer 112 can cause light scattering and/or light intensity attenuation, illustrated as scattered light 160 and reflected light 165, respectively, that reduces the contrast ratio and/or optical efficiency of such displays. For example, optical efficiency can be compromised because AM-TFT structures generally include a series of dielectric layers composed of several different materials suited to insure proper operation of the transistors. These layers can number upwards of a dozen and each one may have a distinctly different refractive index based on the material out of which it is composed. Consequently, the particular ordering of refractive indices at specific layer thicknesses can undesirably cause some of the light entering one (or more) of the layers to be reflected back into the waveguide 130 (e.g., reflected light 165), absorbed, and/or scattered as non-TIR light (e.g., scattered light 160). The layers can cause light attenuation by reflecting much of the light (e.g., reflected light 165) back towards the waveguide, thereby preventing such light from being ejected toward the viewer during pixel actuation. In addition, the various layers of TFTs 114 and conductive interconnects 116 that may have canted surfaces with respect to the slab waveguide surface 133 and with respect to the normal (orthogonal) to that same surface, can cause undesirable scattering from such regions, thus lifting the noise floor and reducing the effective signal-to-noise ratio (and thus the contrast ratio) of the display system. Thus, certain display architectures wherein the driver electronics (e.g., interconnects, TFTs) are generally disposed on the surface of the slab waveguide 130 and/or within layer(s) disposed thereon (e.g., electronics layer 112), and thus perpetually in the path of TIR light 145 traveling inside the waveguide 130, can cause deleterious optical effects by degrading light output or inducing excess scattering that raises the system noise floor.
Another source of optical noise is light that is scattered off of spacers 110 due to the presence of an evanescent field that is generated extending from the top surface 125 when TIR light 145 travels inside the electronics layer 112 and the waveguide 130. Where the evanescent field (i.e., evanescent waves) encounters the spacers 110 resting on the top surface 125, the spacers create a discontinuous boundary condition that causes the evanescent waves to transform into a propagating wave in the visible spectrum but typically at non-TIR angles (i.e., scattered light). This causes a deleterious optical effect by lifting the noise floor and reducing the effective signal-to-noise ratio (and thus the contrast ratio) of the display system.
The disclosed subject matter is directed to addressing the effects of one or more of the problems set forth above.